1. Field of the Invention
The present invention relates generally to a supported silver catalyst useful for the vapor-phase oxidation of ethylene to ethylene oxide. More particularly, the present invention relates to a method of preparing an improved supported silver catalyst post impregnated with cesium.
2. Related Art
The use of supported silver catalysts for the oxidation of ethylene to ethylene oxide has been long known in the art. Additionally, over the years various promoting metals have been added to further enhance performance. In particular, the use of alkali metals has been disclosed in various amounts and added by different methods. A very extensive review of the patent literature is given in G.B. No. 2,043,481A. Such disclosures have been somewhat inconsistent in their teachings, as can be seen by comparing U.S. Pat. No. 2,238,474 in which sodium and lithium hydroxides were suggested as promoters and potassium and cesium were shown to be poisons to U.S. Pat. No. 2,671,764 where rubidium and cesium sulfates were suggested as promoting compounds.
Although alkali metals were suggested generally in the earlier disclosures, it is also generally true that more recent workers in the field have considered potassium, rubidium, and cesium as the preferred alkali metals. For example, see the series of patents to Nielson, et al., in which these materials were used in small amounts co-deposited with the silver--U.S. Pat. Nos. 3,962,136; 4,010,115, and 4,012,425. Still more recently the art has emphasized synergistic combinations of the alkali metals. For example, see G.B. No. 2,043,481A cited above and U.S. Pat. Nos. 4,212,772 or 4,226,782. The art teaches, in addition, that the alkali metals may be used to rejuvenate used catalysts, as for example U.S. Pat. Nos. 4,123,385; 4,033,903; 4,177,169; and 4,186,106. The art teaches that the alkali metals may be deposited either before the silver is placed on the support (pre-deposited)--U.S. Pat. No. 4,207,210; at the same time the silver is deposited (co-deposited)--U.S. Pat. Nos. 4,066,575 and 4,248,741; or subsequent to deposition of the silver (post-deposited)--G.B. No. 2,045,636A.
The amount of alkali metal was suggested to be in quite a wide range in the older art. It was often indicated that large quantities, e.g. up to several per cent of an alkali metal could be used. More recently, the art generally has taught that small quantities of alkali metals produce the optimum effect no matter when the silver and the alkali metals were deposited. Kilty in U.S. Pat. No. 4,207,210 related the optimum amount of alkali metal to the surface area of the support. Exceptions to the above include patents issued to ICI which teach the use of large amounts of sodium alone (G.B. No. 1,560,480) and potassium in combination with smaller amounts of rubidium and cesium (U.S. Pat. No. 4,226,782). However, the art generally teaches that the optimum will be found in substantially lower quantities, perhaps on the order of 50-500 ppm by weight.
It has long been recognized that the method of preparing the catalyst affects its performance. The differing heat "reactivations'" bear witness to this. Additionally, the impregnating solutions used and the intermediate steps have been found to effect the final catalyst. For example, Winnick in commonly assigned U.S. Pat. No. 4,066,575 discloses an impregnating solution containing silver lactate, lactic acid, barium acetate, hydrogen peroxide and water. As a class the lactate based catalyst are very stable but exhibit low selectivity. The support is impregnated with the solution and then first activated by heating in an inert atmosphere at 350.degree. C. for and then dried in air at 200.degree. C. for 12 hours. The "activated" catalyst is then impregnated with a cesium solution and dried in air at 130.degree. C. for 3 hours. The use of the inert atmosphere during the activation step produced a catalyst that was more selective, but much less stable, i.e., the catalyst lost its activity fairly quickly resulting in shorter run length for a given end of run temperature.
Armstrong, in commonly assigned U.S. Pat. No. 4,555,501 disclosed using an impregnating solution containing the silver salt of a neo acid. The impregnated support was then "activated" at temperatures of about 200.degree. C. to 600.degree. C. in the presence of air or reduced oxygen atmospheres, the presence of some oxygen being desirable. The alkali metal, if desired, was then deposited in small quantities (in the range of 260 wppm).
Cesium now appears to be the preferred alkali metal. Various sources of cesium are catalogued in the prior art, for example, cesium hydroxide, cesium nitrate, cesium chloride, cesium chlorate, cesium bicarbonate, cesium carbonate, and other anion functionalities such as formates, acetates and the like.
U.S. Pat. No. 4,374,260 discloses the coprecipitation of silver and cesium salt, such as the carbonate from a silver carboxylate/amino complex.
U.S. Pat. Nos. 4,3350,616 and 4,389,338 both show the deposition of CsCO.sub.3 on to activated silver catalyst from alcohol solution where the silver was derived from aqueous silver salt solution.
U.S. Pat. Nos. 4,066,575 and 4,033,903 disclose the preparation of silver catalyst from both aqueous and non aqueous salt solutions and subsequent treatment of the activated silver catalyst with post deposition of an alkali metal salt such as cesium and anions from lower alcohol and preferable from aqueous solutions. Similarly U.S. Pat. No. 4,342,667 discloses the post deposition of cesium on to silver catalyst derived from aqueous solutions.
What is most clear is from the prior art relating to post deposition alkali metal is the general interchangeability of aqueous and non aqueous procedures, i.e. silver catalyst may be prepared by either aqueous or non aqueous procedures and the post deposition of alkali metal may be aqueous or non aqueous. Furthermore, the salt of silver or alkali metal is not specific. Generally the procedures tended to favor the presence of water.
It has now been found that water at any stage and in any amount is detrimental to the performance of the final catalyst. Thus, the present preparation is characterized as being substantially anhydrous with post disposition of alkali metal, e.g. cesium.
It is an advantage of the present invention that catalysts of exceptional stability in use for the preparation of ethylene oxide are produced, which have high selectivity at high conversions for the ethylene oxide process.